The efficient production of fuels and plastics from renewable sources is one of the key technological challenges of this century (Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek, G.; Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallet, J. P.; Leak, D. J.; Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T. Science 2006 311 484-489). Conventional, petroleum based, high density tactical fuels such as JP-10 and RJ-5 (Diagram 1, exo-tetrahydrodicyclopentadiene and norbornadiene dimers, respectively) will be particularly hard to replace given their high densities of 0.94 g/mL and 1.08 g/mL, respectively. In the case of RJ-5 (perhydroinorbornadiene), significant ring strain contributes to a high heat of combustion (Table 1). Although bulk agricultural waste products such as cellulose and lignin are often targeted as feedstocks for the production of renewable fuels, even saturated hydrocarbon fuels which have previously been prepared from the dehydration products of cellulose derived alcohols have a density of only 0.78 g/mL, (Wright, M. E.; Harvey, Benjamin G.; Quintana, R. Energy and Fuels 2008, 22, 3299) while JP-5 which contains aromatic compounds typically has a density of 0.83 g/mL. These lower densities are reflected in the volumetric heating value of these fuels, with cellulose based jet or turbine fuels and JP-5 capable of producing only 34.3 MJ/L and 34.8 MJ/L, respectively, compared to 39.6 MJ/L for JP-10 and 44.9 MJ/L for RJ-5 (Wright, M. E.; Harvey, Benjamin G.; Quintana, R. Energy and Fuels 2008, 22, 3299) (Burdette, G. W.; Lander, H. R.; McCoy, J. R. J. Energy 1978, 2, 289-292).

TABLE 1Selected data for high density military tactical fuelsJP-5JP-10RJ-5Heating Value, MJ/L34.839.644.9(Btu/gal)(125,000)(142,000)(161,000)Freezing Point, K<227<194>255Specific Gravity (g/mL)0.830.941.08
In addition to having an outstanding volumetric heating value, tactical fuels must have low freezing points for use at high altitudes and in cold climates. These restrictions have limited the use of RJ-5 as a standalone fuel notwithstanding its impressive heating value. Based on these challenging requirements, it is clear that specialized, readily available, and reactive renewable feedstocks will be necessary to produce tactical fuel replacements. α- and β-pinene (Diagram 2) are versatile natural products that are produced by a wide variety of trees and other plant life. They have industrial applications as solvents, pharmaceutical synthons, and in the production of cosmetics and perfumes. Natural turpentine is composed primarily of α- and β-pinene (Coppen, J. J. W.; Hone, G. A. Gum Naval Stores: Turpentine and Rosin from Pine Resin, FAO: Rome 1995). Terpenes have a rich history in the use of pharmaceuticals and have been used themselves as therapeutic agents (Monteiro, J. L. F.; Veloso, C. O. Topics in Catalysis 2004, 27, 169) (Wiegers, W. J.; Hall, J. B.; Hill, I. D.; Novak, R. M.; Schmitt, F. L.; Mookhersee, B. D.; Shu, C.; Schreiber, W. L. U.S. Pat. No. 4,165,301 1979). Due to their compact structures and reactive olefin functionalities, pinenes have significant potential as feedstocks for high density renewable fuels (Harvey, B. G.; Wright, M. E.; Quintana, R. L. Preprints of Symposia-ACS Div. Fuel Chem. 2009 54 305-306) (Filley, J.; Miedaner, A.; Ibrahim, M.; Nimlos, M. R.; Blake, D. M. J. Photochem. Photobio. A 2001 139, 17-21). Both molecules have bicyclic structures incorporating cyclobutanes that possess on the order of 100 kJ/mol of ring strain (Joshi, R. M. J. Macrom. Sci. Part A 1972 6, 595-629). This energy is released upon combustion resulting in a higher heat of combustion than unstrained or linear molecules with similar molecular weights. The volumetric heat of combustion can be further improved through dimerization which significantly increases the density of the mixture. In industry, the dimerization of olefins is often carried out with environmentally unfavorable catalysts such as sulfuric or hydrofluoric acid. These catalysts are corrosive, dangerous to work with, and their use
results in the production of large amounts of waste that must be either treated or recycled, resulting in significant energy demands and higher costs. In contrast, the use of solid acid catalysts provides several advantages over conventional liquid acid systems. For example, these heterogeneous catalysts are typically much less corrosive, safer to work with, easier to separate, and more easily recycled than liquid acid systems (Sheldon, R. A.; Downing, R. S. Applied Catalysis A 1999 189, 163-183) (Kumar, P.; Vermeiren, W.; Dath, J.; Hoelderich, W. F. Energy Fuels 2006 20, 481-487).
Dimerization of α- and β-pinene has been reported utilizing both Bronsted acid catalysts such as phosphoric acid (Phillips, C. F.; Booth, J. W. U.S. Pat. No. 5,723,709 1998) and Lewis acid catalysts such as BF3 (Chapaton, T. J.; Capehart, T. W.; Linden, J. L. Tribology Transactions 2006 49, 454-472) (Chapaton, T. J.; Capehart, T. W.; Linden, J. L. U.S. Pat. No. 6,828,283 2004). Upon hydrogenation, these dimers have been utilized for an array of end uses, from beauty products to traction fluids. Unfortunately, previous studies have revealed complicated product distributions and have provided little evidence as to the structures of the dimers. To selectively produce dimer mixtures with potential uses as high density renewable fuels utilizing more environmentally friendly catalysts, or at least catalysts potentially less environmentally damaging than conventional liquid acid systems, we have studied the reactions of β-pinene with acidic heterogeneous catalysts including Montmorillonite K10 (MMT-K10), Amberlyst-15, and Nafion NR-50.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the invention, as claimed. Further advantages of this invention will be apparent after a review of the following detailed description of the disclosed embodiments, which are illustrated schematically in the accompanying drawings and in the appended claims.